Method of making a scintillator waveguide

ABSTRACT

The present invention is an apparatus for detecting ionizing radiation, having: a waveguide having a first end and a second end, the waveguide formed of a scintillator material wherein the therapeutic ionizing radiation isotropically generates scintillation light signals within the waveguide. This apparatus provides a measure of radiation dose. The apparatus may be modified to permit making a measure of location of radiation dose. Specifically, the scintillation material is segmented into a plurality of segments; and a connecting cable for each of the plurality of segments is used for conducting scintillation signals to a scintillation detector.

This invention was made with Government support under ContractDE-AC06-76RLO 1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights to the invention.

This application is a continuation-in-part of application Ser. No.08/455,586 filed May 31, 1995, now U.S. Pat. No. 5,704,890.

FIELD OF THE INVENTION

The present invention relates generally to a scintillator waveguide forsensing and/or measuring radiation delivery. More specifically thescintillator waveguide has an activator therein. The invention isespecially useful for measuring therapeutic radiation delivered to apatient.

BACKGROUND OF THE INVENTION

Radiation is widely used for medical treatment. The primary radiationsource is gamma or high energy x-rays. Thermal, epithermal, and highenergy neutrons are also used.

It is critical that the amount and location of the radiation delivery becontrolled as closely as possible. An error in intensity can eitherresult in excessive tissue damage, or result in not accomplishing itsintended purpose. An error in location can inadvertently cause damage tohealthy tissue and organs, sometimes to critical organs such as eyes,brain, glands, etc, and cause severe debilitating damage and even death.

One method of estimating dose to a patient receiving thermal andepithermal neutron therapy is to insert a gold needle into the patientand then perform a partial exposure. The partial exposure is typicallycalculated to terminate at about the half-way point. The dose isgenerally terminated by the operator at the half-way point by turningoff the source of radiation, or by closing off the radiation source. Atthat time the gold needle is removed and a radiation count is taken anddose calculations performed. The activation of the gold needle isassumed to be proportional to exposure to the gold needle. From thecount rate taken on the gold needle the estimated dose received by thepatient is calculated. Then the operator continues the radiationexposure to the full prescribed dosage. A disadvantage of this method isthe invasiveness. In addition, gold needles may not be suitable forcertain types of radiation.

Another method of estimating patient dose is with a stimulated phosphor.Disadvantages of using a stimulated phosphor include use of an infraredlaser or other infrared light source to heat the phosphor. Since aninfrared source is required, real-time measurement is not practical.

Another approach to estimating dose is the use of a phantom prior toirradiation. A phantom is in essence a model made of "equivalentmaterial" to the object to be examined. A limitation of this method isthat it is indirect and is subject to variation in the patient(s)compared to the phantom.

Accordingly, there is a need for an apparatus for measuring anon-invasive, real-time actual patient radiation dose.

SUMMARY OF THE INVENTION

The present invention is an apparatus for detecting a ionizingradiation, having:

a waveguide having a first end and a second end, the waveguide formed ofa scintillator material wherein the therapeutic ionizing radiationisotropically generates scintillation light signals within thewaveguide. This apparatus provides a measure of radiation dose.

The apparatus may be modified to permit making a measure of location ofradiation dose. Specifically, the scintillation material is segmentedinto a plurality of segments; and a connecting cable for each of theplurality of segments is used for conducting scintillation signals to ascintillation detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Is a cross section of a probe according to the presentinvention.

FIG. 2: Is a cross section of a waveguide having a cladding and aprotective sheath.

FIG. 3: Is a block diagram illustrating the components of the probeassembly and components used for signal analysis.

FIG. 4: Is a cross-section of a probe having two waveguides.

FIG. 5a: is a cross-section of a probe using a commercial fiber opticconnector.

FIG. 5b: is a cross-section of a probe with a clad waveguide.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, the apparatus of the present invention fordetecting ionizing radiation has a first waveguide 100 having a firstend 102 and a second end 104, the first waveguide 100 formed of ascintillation material wherein the therapeutic ionizing radiationisotropically generates scintillation light signals within the waveguide100. The scintillation material may be organic, for example plasticincluding but not limited to polyvinyltoluene, polystyrene, or mixtures.The plastic may be made with or without wavelength shifters, orinorganic, for example lithium silicate glass, and halide scintillators,for example CsI and/or BaF₂.

In a preferred embodiment, the scintillation material is doped with anactivator selected from the group consisting of ₃ Li⁶, Ce⁺⁺⁺ andcombinations thereof. Epithermal neutrons are preferably detected withglass doped with ₃ Li⁶, or with lithium silicate glass doped with lightemitting Ce⁺⁺⁺ ion.

Cerium-activated ₃ Li⁶ -loaded glasses are fabricated by melting the rawmaterials under a reducing atmosphere control to prevent the formationof Ce⁴⁺. Any reducing atmosphere may be used including but not limitedto carbon monoxide mixed with carbon dioxide, preferably a non-explosivemixture of CO and CO₂, wet hydrogen (i.e. hydrogen gas with watervapor), preferably saturated hydrogen, argon with 4 vol % hydrogen, andmixtures. The glass melt is quickly cooled by pouring onto a chilledmetal plate. This glass can be made into a scintillating fiber form byremelting shards and drawing the fiber from the melt, coating the freshfiber with an appropriate organic resin and polymerizing the resin onthe fiber. Successful fabrication of neutron-sensitive scintillatingglass is critically dependent upon control of the oxidation state duringthe glass melt and fiberizing processes.

When using the present invention to detect the therapeutic dose ofgammas and x-rays, the preferred scintillation material depends upon theintensity of radiation to be measured. For large intensity radiation(above about a few rem/minute), the lowest possible Z number should beused to minimize the production of Compton electrons which can causedamage to the patient. A high Z material increases the production ofCompton electrons with increasing radiation level. For small intensityradiation (below about a few millirem/hr), the use of high Z numberscintillator material improves the detection efficiency. Thus, formaximum detection efficiency, it is preferred to select the highestpossible Z material that may be tolerated under a medical protocol. Formedium intensity radiation, the choice of scintillator material may begoverned by other considerations, for example light output of thescintillator materials and other optical parameters.

Because neutron events produce approximately an order of magnitude morephotoelectrons than gamma-ray interactions, a threshold may be chosen toseparate neutron events from gamma and other radiation events.

In a preferred embodiment, the first waveguide 100 has a coating 106encasing the first waveguide 100, thereby forming a probe 108. Thecoating 106 is preferably a polymer to provide a low activationpotential in a radiation field. Polymers specifically preferred are fromthe group of polysilicones, polypropylenes, polyethylenes andcombinations thereof. The first end 109 may be of any shape, but may betapered as shown for use of the probe 108 as an insertion probe.

The waveguide 100 scintillation material produces light signals that maybe transmitted over long distances. Accordingly, light signals mustpropagate through the scintillation material and any optical cable 110.To achieve long distance transmission of light signals, the light mustbe fully contained within the scintillation material and the opticalcable 100. This containment is achieved in practice via total internalreflection.

Total internal reflection of the light signals of a given wavelengthrange occurs when two conditions are met: 1) the refractive index of thematerial surrounding the scintillation material and/or optical cable 100is smaller than the refractive index of the scintillation materialand/or optical cable 100 itself, and 2) the angle of the light signal issmaller than some angle which is determined by the two refractiveindices. When these two conditions are met in a waveguiding geometry,i.e., the refractive index of the core is greater than that of thecladding, the radiation will propagate with loss only determined by theabsorptive properties of the two materials. In selective circumstances,the cladding may be air, but in most cases, it is necessary andpreferred to surround the waveguide 100 scintillation material and theoptical cable 110 with a coating or cladding 106, 111 with anappropriate refractive index. In some cases, it may be desirable toprovide a cladding 106 that is further surrounded by a protective sheath112 as shown in FIG. 2.

The first waveguide 100 may be attached to a first optical cable 110attached to the second end 104 with an optical interface 114 at thecontact between the second end 104 and the first optical cable 110.

At the optical interface 114 the light signals will either enter thefirst optical cable 110 or be reflected at the optical interface 114.The reflection is minimized when the difference between the refractiveindices of the scintillator material and the first optical cable 110 isminimized and when the quality of the interface is maximized. The lightsignal entering the first optical cable 110 will be transmitted withoutreflective loss only if the ray angles in the second fiber meet thecondition for total internal reflection. Therefore, it is important tothe successful practice of the invention that the refractive indices ofthe scintillating material of the waveguide 100 and the first opticalcable 110 be properly matched.

The opposite end of the first optical cable 110 is preferably connectedto a first port on a scintillation detector (not shown) for receivingoptical scintillation from the first waveguide 100. The first opticalcable 110 preferably has a protective outer sheath or coating 111.

The scintillation detector may be any scintillation detector whichconverts the optical signal to an electrical signal including but notlimited to photomultiplier tube, and avalance photodiode. When using aphotomultiplier tube for measuring neutrons, the photomultiplier tube ispreferably operated at a negative high voltage of about 1000 V. Thephotoelectron signal is pick off the last dynode with a high-speedelectronics circuit. The high-speed electronics circuit has apreamplifier connected to a 60-ns integrator which is followed by adiscriminating and pulse-counting circuit. The discriminating circuithas a threshold set at least about 1.25 photoelectrons to minimizephotomultiplier tube dark count and gamma-ray sensitivity. In a reactorenvironment, the threshold needs to be set higher for the intensegamma-ray fields therein.

The apparatus of the present invention may be used for determining therelative location with respect to the patient of the therapeuticradiation as well as a dose level. In FIG. 4, in addition to the firstwaveguide 100, a second waveguide 400 is added. In order to distinguishbetween scintillation from the first waveguide 100 and the secondwaveguide 400, the second waveguide 400 is constructed to scintillate adifferent color light compared to the first waveguide 100. Differentcolor light scintillation is achieved by using different activators, forexample a rare earth, (e.g. cerium (Ce⁺⁺⁺), erbium) in glasses orcrystals; sodium or thallium in alkali halide crystals such as cesiumiodide; or different wavelength shifters, for example,3-hydroxy-flavone. The scintillation detector 300 or 304 must then beable to distinguish between light signals of the different colors.

EXAMPLE 1

An experiment was conducted to demonstrate radiation detection of thepresent invention. Two probes were constructed as shown in FIG. 5a, 5b.In FIG. 5a, the first waveguide 100 was mounted in a commercialfiber-optic connector 500. The commercial fiber-optic connector 500 hasan outer housing 503 that holds two inner sleeves 505 into which areinserted the first waveguide 100 and the first optical cable 110. Epoxy507 was used to secure the first waveguide 100 and the first opticalcable 110 in the inner sleeves 505. In FIG. 5b, the first waveguide 100with its cladding 502 is glued with silicon into a first glass capillary504 as a waveguide assembly. The waveguide assembly is glued within asecond capillary 506 together with an adjoining commercial clad opticalfiber 110. The second capillary 506 extends over the commercial opticalfiber 110 and any annular space therebetween is filled with silicon 508.

Fluxes typical of boron neutron capture therapy (about 10¹⁹ n/cm² /sec)and an epicadmium flux have been measured with both probes for periodsin excess of an hour. The response of the waveguide 100 was linear withreactor power at the higher (≧10 kW) reactor powers. At lower powers(<10 kW), the residual gamma-ray flux was large enough that the detectorresponse was not linear with reactor power. Even though the pulse-heightthreshold was not optimized for gamma-ray rejection, the gamma-rayinterference was still quite small. The gamma-ray sensitivity would bereduced by more than an order of magnitude by increasing the thresholdbias by 50%.

No degradation was observed after leaving a waveguide 100 in the reactorgamma-ray field for 16 hours.

CLOSURE

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

We claim:
 1. A method for making a waveguide for radiation detection,comprising the steps of:(a) melting a scintillator material and anactivator selected from the group consisting of ₃ Li⁶, Ce⁺⁺⁺ andcombinations thereof in a reducing atmosphere in a doped scintillatormelt; (b) forming the doped scintillator melt into a waveguide bypouring the doped scintillator melt onto a chilled metal plate andforming shards then remelting the shards and drawing the waveguide fromthe melted shards.
 2. The method as recited in claim 1, furthercomprising the step of coating the waveguide with an organic resin andpolymerizing the organic resin.
 3. The method as recited in claim 1,wherein said reducing atmosphere is selected from the group consistingof carbon monoxide mixed with carbon dioxide, wet hydrogen, argon with 4vol % hydrogen, and mixtures.